Discovery of CWISE J052306.42-015355.4, an Extreme T Subdwarf Candidate

We present the discovery of CWISE J052306.42-015355.4, which was found as a faint, significant proper-motion object (0.″52 ± 0.″08 yr^-1) using machine-learning tools on the unWISE re-processing of time series images from the Wide-field Infrared Survey Explorer. Using the CatWISE2020 W1 and W2 magnitudes along with a J-band detection from the VISTA Hemisphere Survey, the location of CWISE J052306.42-015355.4 on the W1 - W2 versus J - W2 diagram best matches that of other known, or suspected, extreme T subdwarfs. As there is currently very little knowledge concerning extreme T subdwarfs we estimate a rough distance of ≤68 pc, which results in a tangential velocity of ≤167 km s^-1, both of which are tentative. A measured parallax is greatly needed to test these values. We also estimate a metallicity of -1.5 < [M/H] < -0.5 using theoretical predictions.